This invention relates generally to a method for removing vapor-phase contaminant compounds from a gas stream. The method treats exhaust gases from a combustion or industrial process to remove gas phase contaminants such as mercury and trace metal and organic compounds, and other vapor compounds that are not readily removed by conventional exhaust gas treatment processes. This invention may be applied to treat flue gases from coal or oil-fired boilers, incinerator flue gases, and other sources of gas-phase environmental contaminants. Specifically, the invention relates to a method of injecting a slurry of a fine adsorbent material and chemical additives into an exhaust gas stream to enhance adsorption of gas-phase contaminants onto the resulting particles that are then readily collected by conventional means.
Control of atmospheric emissions from industrial and power generating processes has long been recognized as an important and often complex problem. Environmental standards for emissions from manufacturing and combustion sources such as petroleum and chemical refineries, incinerators, metal extraction operations, and power plants are becoming increasingly stringent. Title III of the 1990 United States Clean Air Act Amendments requires major emission sources to control emissions of air toxics to less than 10 tons per year of any one species and less than 25 tons per year for all species. The U.S. Environmental Protection Agency (EPA) is developing proposed mercury regulations which may be even more stringent and plans to issue final regulations on mercury emissions from coal-fired boilers by Dec. 15, 2004 with full compliance to be required by December, 2007.
Air toxics present in the flue gases of combustion sources are typically present in both the particulate and gas phases. The particulate phase includes both fly ash and particulate metals such as nickel, arsenic, and chromium. More volatile metals, such as mercury and selenium, as well as organic compounds and halides, tend to partition between the vapor phase and the fly ash. The phase distribution of volatile and semivolatile contaminants in exhaust gases is a generally complex function of a number of factors including temperature and the chemistry of both the contaminant compound and the particulate phase.
Mercury has long been known as an important health and environmental hazard. As such, a number of techniques have been developed to remove mercury from gas streams. These include injecting dry sorbents into the gas stream before removal of particles in a particulate collection device, passing the gas stream through a fixed sorbent bed, or using a wet scrubbing method to absorb soluble mercury species. Sorbent injection mercury control typically uses activated carbon or some comparable carbon-based sorbent that typically has particle sizes in the range of 10 to 40 xcexcm. The powdered sorbent is dispersed into the duct work of an exhaust gas flow system as a dry powder via a tube fed by a pneumatic transport system or other similar apparatus. Particles in the range of 10 to 40 xcexcm are small enough to become entrained in rapidly moving air such as in a flue gas stream. After a short contact timexe2x80x94approximately 1 to 5 secondsxe2x80x94particles are removed from the flue gas by means of a conventional particle collection system such as a baghouse filter or an electrostatic precipitator before the gas is exhausted into the environment. Cost estimates indicate that commercialization of this contaminant control method for cleaning the flue gas of coal fired electric power plants could result in a five percent increase in electricity prices and that 95 percent of the increase would be due to the cost of the activated carbon consumed in some cases.
A key limitation of the aforementioned sorbent-based control method for vapor-phase contaminant control is the mass transfer rate, the amount of mercury contacting the sorbent surface over time. One of the determining factors for mass transfer is sorbent particle size. Modeling calculations and actual tests of mercury removal in coal-derived flue gas by sorbent injection demonstrate that smaller sorbent particle sizes increase removal efficiencies for mercury and other gas-phase-contaminants. The smaller the sorbent particle size for a given mass of sorbent, the better the mass transfer and therefore the better the mercury adsorption effectiveness. However, simply grinding larger particle size activated carbon or other sorbent powders into smaller particles is not a straightforward process. There is an increased energy cost associated with formation of smaller particles, and this cost increases dramatically for particle sizes less than approximately 5 xcexcm. Activated carbon particles as small as 2 to 3 xcexcm in effective diameter may be formed through mechanical grinding, but this process becomes cost prohibitive from an energy standpoint. Additionally, dry grinding to form particles of this size frequently leads to severe dust problems during preparation, handling, and use of the activated carbon. Storage and feeding of very fine dry powders is further complicated by caking and clumping issues.
In some flue gases, vapor-phase mercury is not effectively removed by activated carbon because the mercury species present do not adsorb well on the activated carbon surface. Under these conditions, the sorptive capacity of the activated carbon may be improved through the use of special chemical additives such as sulfur and halide compounds impregnated into the activated carbon before injection into the flue gas. Prior art methods of preparing activated carbon in this manner employ additional processing and handling steps which can add significant cost to the process.
Moller et al. (U.S. Pat. No. 4,889,698) disclose a method for removal of mercury and gas-phase organic compounds, such as chlorodibenzodioxins and chlorodibenzofurans, from flue gases that employs injection of powdered activated carbon into the flue gas in conjunction with a spray absorption process using alkali sorbents for removal of acid gases. The spray absorption step of the disclosed method serves to cool the exhaust gas and lower the vapor pressure of the vapor-phase contaminants, thereby enhancing adsorption and condensation of vapor-phase contaminants on fly ash particles and the injected activated carbon particles, as well as improving the removal of acid gases on the alkali components. In this prior art system, the powdered sorbent particles are prepared and added separately from the liquid injection step. Any grinding that is required to produce particles of the desired size is done by a dry method.
An additional prior art reference by Moller et al. is provided in a patent application published by the European Patent Office (Application No. 86305054.8, published Jan. 14, 1987). This application discloses a method for treating flue gases to remove vapor phase mercury and chlorinated dioxins and furans. The method comprises xe2x80x9cincorporation of a relatively small amount of activated carbon in an alkaline feed suspensionxe2x80x9d that is sprayed into the flue gas. A small amount of activated carbon is added to a feed suspension of alkaline solid components such as calcium hydroxide. This suspension is sprayed into the flue gas with the primary intention of removing acid gases such as sulfur dioxide. Evaporation of the aqueous suspension produces airborne particles. The activated carbon used in the disclosed method is in much lower concentrations than the alkali components and the resultant particles are characterized by Moller et al. as activated carbon xe2x80x9cembedded in the particulate matter comprising the reaction products of e.g. calcium hydroxide and the acidic components of the gas.xe2x80x9d This co-injection of lime (calcium hydroxide) and carbon, for the separate removal of acid gases and vapor phase toxins leads to nearly complete coating of the activated carbon with acid gas-alkali reaction products. The resultant carbon particles are substantially reduced in usable adsorptive surface area as the coating of lime or other alkaline components blocks access to internal micropores.
In recent full-scale testing, it has also been demonstrated that the co-injection of an alkali with activated carbon further reduces the effectiveness of the activated carbon for mercury as the alkali removes key acid gas components, such as hydrochloric acid, that enhance effective mercury removal on activated carbon. Additionally, the methods disclosed in the two Moller et al. references both induce and depend on a substantial temperature drop (50 to 100xc2x0 F.) in the flue gas to provide effective acid gas removal. Reduced flue gas temperatures tend to drive gas-particle phase partitioning toward the condensed or adsorbed phase which improves vapor-phase contaminant removal. This approach requires large amounts of water and large spray dryer towers that may increase capital and operating costs substantially and also lead to increased corrosion in the equipment and resulting increases in maintenance costs. Furthermore, other flue gas treatment steps downstream of the sorbent treatment system, such as for instance catalytic nitrogen oxide destruction, may require higher operating temperatures that necessitate reheating of the flue gas, thereby further increasing operating costs. Moller et al. do not teach wet grinding of the activated carbon to achieve small sorbent sizes nor the addition of chemicals to enhance the activated carbon effectiveness.
It is an object of the present invention to provide a method that overcomes many of the aforementioned problems with prior art sorbent injection systems and methods. Advantages of the present invention include a simpler, more efficient method and system for removing vapor phase contaminants such as mercury, organic compounds, and toxic metals from a gas stream such as flue gas from a fossil fuel fired power plant.
The objectives and advantages of the present invention are achieved by a method for converting difficult to collect vapor-phase contaminants in an exhaust gas stream to the airborne particle-phase so they can be more readily removed by conventional particulate air pollution control technologies.
In one embodiment of the present invention, a method is provided for removing one or more vapor-phase, adsorbable contaminants from a gas stream. A sorbent material is wet ground to form a slurry of fine sorbent particles in a solution of one or more solvents. This slurry is atomized into the gas stream to form a plurality of slurry droplets. The solvents rapidly evaporate from the slurry droplets upon encountering the gas stream to produce a dispersed aerosol of fine sorbent particles. These dispersed aerosol particles scavenge vapor-phase, adsorbable contaminants from the gas stream before being separated from the gas stream using a particle separation method.
In another embodiment of the present invention, a method is provided for removal of one or more vapor-phase contaminants from a gas stream. A sorbent powder is mixed with one or more solvents to form a slurry in a solution. The slurry also contains one or more dissolved chemical additives that enhance the ability of the fine sorbent powder particles to adsorb or react with the target vapor-phase contaminants. The slurry is atomized into the gas stream to form a plurality of slurry droplets. The plurality of slurry droplets rapidly evaporates in the heated gas stream to produce a dispersed aerosol of fine sorbent particles that are impregnated with the chemical additives that had been dissolved in the slurry solution. The dispersed aerosol particles adsorb the vapor-phase contaminants from the gas stream. Then, the aerosol sorbent particles are separated from the gas stream using a particle separation method.
In a further embodiment, a system is provided for removal of one or more vapor-phase, adsorbable contaminants from a gas stream. The system includes a mixing chamber wherein one or more solvents and one or more sorbent powders are combined to form a slurry. The slurry also contains one or more dissolved chemical additives. The slurry is delivered to the gas stream as a spray of fine droplets via an atomizer. The fine droplets quickly evaporate in the gas stream to form a dispersed aerosol of fine sorbent particles impregnated with the chemical additives. These additive-impregnated sorbent particles adsorb and remove vapor phase contaminants from the gas stream before being removed from the gas stream by a particle separator.
In an alternative embodiment of the present invention, a system is provided for removing one or more vapor-phase, adsorbable contaminants from a gas stream. The system includes a mixing chamber wherein one or more solvents and one or more sorbent powders are combined to form a slurry that contains one or more dissolved chemical additives. A grinder positioned within the mixing chamber grinds the sorbent powder into fine particles of less than approximately 5 xcexcm diameter while mixing the solvents and additives with the fine particles to form the slurry. One or more sorbent hoppers are provided for storing the sorbent powders, and one or more sorbent metering devices is provided for supplying the sorbent powders to the mixing chamber at a controllable rate. One or more solvent source containers is provided for storing said one or more solvents, and one or more solvent metering devices is provided for supplying the solvents to the mixing chamber at a controllable rate. One or more chemical additive reservoirs is provided for storing said one or more chemical additives, and one or more additive metering devices is provided for supplying the chemical additives to the mixing chamber at a controllable rate. The slurry is delivered as a spray of fine droplets via an atomizer into the gas stream. The spray of fine droplets quickly evaporates in the gas stream to form a dispersed aerosol of fine sorbent particles impregnated with the chemical additives. These additive-impregnated aerosol particles adsorb vapor-phase contaminants from the gas phase and are then removed from the gas stream by a particle separator.